A wide assortment of automatic, body-implantable medical devices are presently known and commercially available. The class of such devices includes cardiac pacemakers, cardiac defibrillators and cardioverters, neural stimulators and many others.
In general, cardiac pacemakers are electrical devices used to supplant some or all of an abnormal heart's natural pacing function. Pacemakers typically operate to deliver appropriately timed electrical stimulation signals, sometimes called pacing pulses, designed to cause the myocardium to contract or "beat. "
Known and commercially available pacemakers are typically characterized according the chambers of the heart to which they are capable of delivering stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers, especially early ones, deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly at present, however, pacemakers sense electrical cardiac activity in one or both chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based upon the occurrence and recognition of such sensed intrinsic electrical events.
The Inter-Society Commission for Heart Diseases has established a standard three-letter descriptive code to characterize pacemakers. The first letter of the ICHD code indicates which chambers of the heart the pacemaker is capable of pacing (atrial (A), ventricular (V), or atrial and ventricular (D)). The second letter indicates which chambers of the heart the pacemaker is capable of sensing (A, V, or D), and the third letter indicates the response of the pacemaker to a sensed intrinsic event (triggered (T), inhibited (I), or dual (D)).
A VVI pacemaker, for example, senses electrical cardiac activity only in the ventricle of a patients heart, and delivers pacing stimuli only in the absence of sensed electrical signals indicative of natural ventricular contractions (an "inhibited" response). A DDD pacemaker, on the other hand, senses electrical activity in both the atrium and ventricle of the heart (an "inhibited" response), and delivers atrial pacing stimuli only in the absence of sensed signals indicative of natural atrial contractions, and ventricular stimuli only in the absence of sensed signals indicative of natural ventricular contractions (an "inhibited" response). The delivery of each stimulus by a DDD pacemaker is synchronized with prior sensed or paced events (a "triggered" response, and since the responses are also "inhibited" as noted above, the third letter in the ICHD code is "D").
Many state-of-the-art pacemakers are capable of performing either unipolar or bipolar sensing and of pacing in either chamber of the heart. Unipolar pacing requires an elongate lead having only one insulated conductor therein and only one generally distal electrode disposed thereon. As will be appreciated by those of ordinary skill in the art, in most unipolar configurations, the protective canister of the implanted device is conductive and functions as an electrode in pacing or sensing. In particular, for unipolar pacing the current path for stimulating pulses extends from the pacemaker's pulse generator, along the lead to the exposed distal electrode, through the cardiac tissue, and back to the pacemaker via the conductive canister.
For bipolar pacing and/or sensing, on the other hand, a lead having two mutually isolated conductors and two electrodes disposed thereon is required. Typically, one electrode is disposed at the distal end of the lead and is referred to as the "tip" electrode, while the second electrode is spaced back somewhat from the distal end of the lead and is referred to as a "ring" electrode. The current path for bipolar pacing extends from the pulse generator in the pacemaker, along a first of the two lead conductors to the tip electrode, through the cardiac tissue to the ring electrode, and back to the pacemaker along the second of the two lead conductors.
Typically, a fully-featured DDD pacemaker, since it is capable of sensing and pacing in both chambers of the heart, can be programmed to operate in modes other than DDD, for example, VVD, VVI, AAI, etc . . . . In addition, many state-of-the-art pacemakers can be programmed to operate in either unipolar or bipolar pacing and sensing modes. This gives the implanting physician considerable flexibility in configuring a pacing system to suit the particular needs of a given patient. In addition, since most of today's pacemakers are non-invasively programmable post-implant, the physician or clinician can re-program the already implanted device to operate in different modes and with different pacing and sensing polarities in response to changes in the patient's needs and condition.
Many pacemakers can accept and will operate with either unipolar or bipolar leads. Thus, it is important for the physician to be aware of which type of leads are used in a given instance, since it would be inappropriate to program the device into a bipolar pacing and/or sensing mode when only unipolar leads have been implanted. Similarly, if one of the two conductors or electrodes on an implanted bipolar lead were to fail for some reason (e.g., breakage of a conductor due to metal fatigue, poor connections between the lead and the pacemaker itself, tissue degradation at the electrode site, subclavian crushing of the lead, metal ion oxidation, short of lead conductors due to urethane/silicon breakdown, etc. . . ) it would be necessary to re-program the pacemaker into unipolar pacing and sensing modes in order for the pacemaker to function properly. The need for such re-programming due to lead failure or improper initial programming, however, would only become apparent upon careful examination of the patient in a clinical setting, which may not occur frequently enough to ensure proper operation of the pacemaker over a long term of implant.